It is possible by means of dental apparatus for x-ray diagnosis as described in U.S. patent application Ser. No. 618,514 filed Jan. 10, 1975 to produce general tooth and jaw x-ray pictures while automatically controlling the x-ray dose output received by the film in accordance with a preset nominal value, so that an x-ray picture is produced with automatic exposure. However, the darkening or image formation on the film depends not only on the x-ray dose output of the radiation acting on the film, but also on the speed at which the x-ray film is transported. Therefore, the apparatus for dental x-ray diagnosis according to the aforesaid Application produces x-ray pictures with optimal darkening of the picture only if the rate of film transport is constant. However, it has been found in practice that variations or fluctuations of said transport speed of the film do occur.